The present invention relates to a driving device for a motor-driven compressor having an electrolytic capacitor in its power unit, and particularly to the discharge of an electrolytic capacitor in an air-conditioning system in household and automobile applications where safety must be assured to protect human bodies from electric shocks, firing, burns, and other hazards.
Conventionally, motor-driven compressors (herein after simply referred to as xe2x80x9ccompressorsxe2x80x9d) have been used mainly for household air-conditioning systems. However, with the recent penetration of electric vehicles, hybrid cars, fuel cell powered vehicles, or the like, an increasing number of compressors are also used for automobile air-conditioning systems. FIG. 7 shows a circuit diagram of a driving device used for a compressor in a conventional air-conditioning system incorporated in an automobile.
In FIG. 7, compressor 501 of the air-conditioning system has a three-phase motor unit and a compressing unit therein. Battery 502 serves as a power source of both compressor 501 and a drive motor, and normally supplies voltages ranging from 100 V to 300 V.
Switch 504 turns on/off the power supplied from battery 502 as required, and is always closed when compressor 501 is operated. The power from battery 502 is supplied to driver 505 via switch 504 and electrolytic capacitor for smoothing current 503. Driver 505 includes a plurality of switching elements for supplying power to compressor 501 and base (gate) drive circuit 511 for driving the elements.
Driver 505 performs inverter control and pulse width modulation (PWM) on DC voltages supplied from battery 502 thereby to convert them into pseudo AC voltages (sine wave) formed by positive and negative rectangular pulses. Then, the driver changes the voltages or frequencies to control the number of revolutions of compressor 501. The values specifying the numbers of revolutions are given from air-conditioning controller 506 that controls the entire air-conditioning system. Controller 506 determines the numbers of revolutions of compressor 501 so that the inside of the automobile is always kept comfortable in accordance with its environmental conditions, and sends the specified values to driver 505.
Hereinafter described is how capacitor 503 discharges for a conventional driving device after the operation of compressor 501 is stopped and switch 504 is opened.
During the operation of compressor 501, driving current flows through driver 505. On the other hand, even when compressor 501 is at rest, some amounts of current flows. That is, driver 505 includes a microcomputer for control and various protection networks therein and such circuits carry a small amount of current (hereinafter referred to as xe2x80x9cdark currentxe2x80x9d), though it is weaker than the driving current for the compressor.
Capacitor 503 is discharged by this dark current after compressor 501 has stopped, and it takes a considerable period of time to discharge the capacitor completely. For example, when the supply voltage is 300 V, the capacitance of the capacitor is 1000 xcexcF, and the dark current is 20 mA and constant, it takes 15 seconds to discharge the capacitor completely. It takes 13.5 seconds to discharge the capacitor to 30 V, which is said to a safety voltage at which human bodies do not get electric shocks in an automobile.
FIG. 8A is a timing chart showing the operations of each component after compressor 501 starts its operation and then stops, and before capacitor 503 completes discharge. Now this timing chart is explained.
At timing A, the instruction from controller 506 is changed from xe2x80x9cStopxe2x80x9d to xe2x80x9cOperatexe2x80x9d. Then, the signal is sent to switch 504 and the switch 504 is closed after time T1 delay. This time T1 delay is an operational delay of switch 504. Upon closure of switch 504, capacitor 503 is charged, output of driver 505 is switched on, and energization to compressor 501 is started.
Next, when the instruction from controller 506 is changed from xe2x80x9cOperatexe2x80x9d to xe2x80x9cStopxe2x80x9d at timing B, switch 504 is opened after time T1 delay, and at the same time, output from driver 505 is switched off and compressor 501 is de-energized. Since the voltage of capacitor 503 after that time depends on the natural discharge caused by the dark current as mentioned above, it slowly decreases over time T4. Thus, capacitor 503 is discharged completely. The time T4 is 15 seconds under the above conditions.
FIG. 8B is a control flow chart of driver 505. Capacitor 503 is naturally discharged by the dark current and no special discharge control is performed on it.
With the recent penetration of electric vehicles, hybrid cars, and fuel cell powered vehicles, or the like, safety measures to protect not only crew but also mechanics engaged in maintenance of such vehicles from high voltages is becoming necessary.
However, with the above-mentioned conventional driving device for a motor-driven compressor, it takes about a dozen seconds to discharge the electrolytic capacitor. During maintenance work of an air-conditioning system, mechanics may misunderstand the system has been stopped and touch the circuits, even though the discharge of the capacitor has not been completed yet. Therefore, the conventional driver for a compressor has a problem that some safety measures must be taken for such a case.
When a capacitor that has not completely discharged yet is short-circuited with tools or the like, sparking occurs. Safety from such a case must be assured. Particularly, since hybrid cars carry gasoline, they require additional assurances of safety. Similarly, since fuel cell powered vehicles, or the like, use hydrogen as a fuel, they also require additional assurances of safety. Moreover, for hydrocarbons (e.g. propane) recently used as a new refrigerant for air-conditioning systems, maximum safety must be assured in the replacement of gas.
In addition, the conventional driver for a compressor has another problem that when an external resistor is installed to discharge the capacitor for a shorter period of time, the resistor carries current and thus increases the loss of the circuit and the size of the system.
The present invention addresses the problems discussed above. It is, therefore, an object of the present invention to provide a small and high-efficient driving device for a motor-driven compressor assuring safety of the crew and mechanics, in an air-conditioning system to be incorporated in an electric vehicle, hybrid car, or fuel cell powered vehicle, or the like, operating from a high-voltage source, and also in an air-conditioning system using flammable refrigerants.
A driving device for a motor-driven compressor of the present invention is comprised of:
(a) a motor-driven compressor for compressing a refrigerant;
(b) a DC power supply serving as a power source of the motor-driven compressor;
(c) a capacitor connected in parallel with the DC power supply;
(d) a switch provided between the DC power supply and the capacitor, and closed when the motor-driven compressor is operated and opened when the compressor is stopped;
(e) a driver for converting electric power supplied from the DC power supply via the switch and the capacitor into driving power for the motor-driven compressor, and for outputting the driving power thereto;
(f) a controller for instructing the driver to operate or stop the motor-driven compressor; and
(g) a discharge control unit provided in the driver and controlling the driver so that the capacitor is discharged using the motor-driven compressor as a medium, after the instruction to stop the motor-driven compressor is given.
The above structure allows the control of the driver so that the capacitor is discharged using the motor-driven compressor as a medium for discharge; thereby realizes a small and high-efficient driving device for a motor-driven compressor with maximum safety.